In recent years, lubricants such as lubricating oil or lubricating grease are used under a severe condition such as high temperature, high speed and high load with downsizing and lightweighting of various machines and bearings, high speed processing and maintenance-free systems. Therefore, lubricants are become indispensable which have superior heat resistance and oxidation resistance.
In bearing lubrication, under high temperature and high-speed condition, a lubricating oil or grease used for lubrication causes a temperature increase due to oil film shortage or heat and oxidative degradation. As a result, a base oil of lubricant is promoted to evaporate to form sludge, damage the bearing and decrease a life of machine.
Therefore, various lubricating oils and greases are investigated which are usable at high temperature condition. Generally, an improvement of the lubricants under a high temperature condition highly depends on a base oil of lubricant which is contained at largest amount. Examples of such heat-resistant base oils are polyphenyl ether, silicone oil and fluorinate oil (for example, non-patent literature 2). However, polyphenyl ether and fluorinated oil are generally very expensive, and silicone oil is said to be generally disadvantageous in lubricity.
It is important to take a balance between cost and performance of the base oil for providing lubricating oils and greases which are low-cost but high-performance. Lubricating oil for high temperatures and heat-resistant grease are known which use alkyl diphenyl ether as a base oil (e.g., HILUBE LB-100, MORESCO Corporation) (for example, patent literatures 1 and 2, non-patent literature 1). Alkyl diphenyl ether is excellent in stability to heat and oxidation, causes particularly a little evaporation in use under high temperature, and small in viscosity variation. Further, when heat-resistant grease containing alkyl diphenyl ether as a base oil is used for a ball bearing and a roller bearing, it is apparent that brittle separation by hydrogen brittleness of metal materials extremely decreases which is caused by invasion of hydrogen to the steel which occurs in fixed rings (for example, non-patent literature 2). Because such a characteristic is hardly available by other general-purpose synthetic lubricants, e.g., α-olefin oligomer, polyalkylene glycols, ester-based oil, a heat-resistant grease lubricating oil which is relatively inexpensive and superior property can be obtained using alkyl diphenyl ether.
With the above superior property, a heat-resistant grease using the alkyl diphenyl ether as a base oil and urea or lithium hydroxystearate as a thickener can be used in a wide temperature range from low to high temperatures, and is widely used for a bearing for automobile electric auxiliary machine such as an alternator which requires long life and long-term reliability.
However, in recent years, these automobile electric auxiliary parts become inferior in thermal dissipation due to downsizing and lightweighting of the parts. Therefore, the heat-resistant grease for bearing is used under more severe condition than usual in view of temperature environment, and more improvement of the base oil is expected.
At present, alkyl diphenyl ether is prepared by an addition reaction of diphenyl ether and a linear α-olefin such as 1-dodecene or 1-tetradecene, and a ratio of the carbon at benzyl position being quaternary is 30 to 40 mole %. This alkyl diphenyl ether is expected to be more improved in heat-resistance.